1. Field of the Invention
The present invention relates to a driving control system for a switching network of an exchange such as a telephone exchange in which semiconductor switch elements are employed for linking the outgoing and incoming speech path lines. More particularly, the invention concerns a driving control system for a multi-stage switching network in which a plurality of switch matrices each including a number of cross-point circuits arranged in a matrix array each of which in turn includes a storage cell for storing therein an information signal for opening or closing the associated cross-point switches in a speech path and a control input circuit for controlling the information signal stored in the storage cell by at least three logical inputs.
2. Description of the Prior Art
The conventional switching network as represented by a telephone exchange employs mechanical-contact cross-point switches such as reed relays and cross-bar switches. With the recent marked development of semiconductor technology, however, a semiconductor cross-point switch has come into practical use. As compared with the hitherto known electromagnetic cross-point switch which requires a high power of several hundred milli-watts to several ten watts for the operation thereof, the control signal for the switching network using the semiconductor elements as the cross-point switches may be a logical pulse signal of a significantly reduced magnitude on the order of several volts or several milli-amperes which permits nevertheless an enhanced high-speed operation of the switching network.
However, the use of the semiconductor cross-point switch elements in turn means that the noise immunity of the switching network is correspondingly lowered, involving possibly erroneous operations.
In general, because the switching network employs an enormous number of cross-point switches arrayed in a predetermined number of matrices each having m rows and n columns (i.e. m.times.n cross-points) which matrices are then grouped into a plurality of stages and interconnected through link connections, a correspondingly large space is required for the installation. Further, a remarkably large number of control line wires is required to selectively drive the cross-point in the switch matrices the number of which usually amounts from several tens to several hundreds. Under these conditions, when a general purpose IC logic circuit such as TTL logical circuits is employed for the control input circuit for receiving a command information signal to close or open the corresponding semiconductor cross-point switch element, the unitary control range of a switch controller for driving and controlling the switching network has to be limited to a small extent in consideration of the required noise immunity as well as the wire length. For example, the switch controller will have to be provided for every switch stage thereby to limit the wire length to less than several ten centimeters. Further, in order to assure a stabilized signal transmission, each of the switch packages has to be provided with a buffer gate circuit for receiving the control signals. It goes without saying that the provision of TTL logic circuit together with a holding circuit such as a flip-flop in correspondence to the cross-point is undesirable from the economical standpoint and also in view of expensive packing processes. In brief, although the switching network employing the semiconductor cross-point switch elements allows the logic pulse signal to be utilized as the control signal, significant reduction in the amount of hardware as well as simplification of the manufacturing process has not yet been attained, which provides an obstacle in promoting economical fabrication of the switching network.